Bounder: Tire Pressure

[Craig36]

My Pace Arrow 36B plate says 100 lbs in both front and rear. That is for an empty coach. It runs OK with 100, but at 102 handles much better and may give a tiny improvement in mileage. I'm sticking with 102. Max rating on my Michelins is 110 lbs.

[jadatis]

Originally Posted by moisheh

Jatadis: I will go along with your offer of help even though I am a skeptic! Tires are 275/70 22.5 H ( 16 ply) rated. Max load is 6945 singles and 6395 duals. Interesting though was that the pressure for those loads is 131 lbs. I bet the wheels are rated for 120 lbs. at most. Short DP's are usually very light but often use the same tires as a 40 ft. of the same make. I am betting that my 90 lbs. will be just fine. We are going on a trip this weekend and I will bring my infrared thermometer and stop a few times to take temps.

I took the reserves so that it gave 90 psi so you can see how much reserves this gives.
Read the picture and take your own conclusions.
Mine is that 90 psi as you expected is not realy bad, but if weighing fully loaded prooves heavyer to low.
found also from Pirelli the sises of tires with higher maximum load and 9.0 bar =131 psi but was for speedcode M is up to 130km/80m/h so yours are probably N speedrated so for up to 140km/87m/h .
Here the link to pirelli truck tires see page 19 and 20 of this pdf.
http://www.pirelli.com/mediaObject/p.../tech_data.pdf
And here the picture of filled in spreadsheet. I used the weighed axle loads you cave in your beginning post, and added a what if weight.
The spreadsheet can be found in this map under the name moishehIRV2 so you can chanche and add it if you have the weighed 4 point loads in part 3.
https://skydrive.live.com/?cid=a526e...E092E6DC%21836

[jadatis]

Saw I made a mistake the tires you have where LI148 so probably for M speedcode= upto 130km/h . 148 for single load is also given on the pirelli document.
The stange 1331 psi is a conversion of 900kPa/ 9.0 bar.
Tires all over the world are calculated in Bar and KG and later conversed to Lbs and psi and mostly then again rounded.

[FastEagle]

Originally Posted by moisheh

Our new to us 2005 34M DP has a 10,500 FA and a 17,500 RA rating. I weighed it when we first took delivery. FA was 7400 and RA was 15,540. Although we had loaded some items it was light at the time. Had full fuel and propane and half tank of water. The scale we used was a Cat scale and they are elevated so no way to do each wheel position. I know from experience with another Bounder that the weight can vary as much as 500 lbs. from side to side. Both slides are on the DS. Fridge and water heater are on the PS. FW wall decal says to inflate to 90 lbs. We have new 275/70 Hankooks and I have the tire chart. Finding the right pressure is a mystery for me. 90 lbs. would support 10,390 on the front and 18740 on the rear. I am sure that when we are loaded the rear will be heavier. Probably more like 17,000 The front would also be heavier.Maybe 8200. I was thinking of 85 lbs. in the fonts ( supports 9570 lbs.) and the same in the rear ( supports 17640). Looking for comments on my logic. What do others run? I am more concerned with the fronts. Need to get a pressure that will support the weight yet not make for a rough ride. 80 lbs. would probably work but I am afraid the tires would run hot.

Thanks

Here are two references to help you understand weights and tire pressures. I know they are not from the manufacturer of your tires but they outline "tire industry standards" that all retailers are required to follow. Read them carefully and apply the information needed to be successful with your new tires.

NOTE: There are other RV tire manufacturers that do not support the weight by heaviest tire position method. That is because there is zero load capacity reserves when only aired to the weight carried.

http://www.tiresafety.com/images/Tir...t%20Manual.pdf
http://www.trucktires.com/bridgeston.../WeighForm.pdf


FastEagle

[moisheh]

I am afraid that some of the info provided in this thread is not only confusing it may be incorrect. Jadatis: I am assuming you are in Europe as I don't think Pirelli sells commercial tires in NA. Regardless none of the 22.5 commercial tires are speed rated at 81 mph!! But I thank everyone for their assistance. I am going to reweigh when loaded. Yesterday I was on the road and the ambient temp was 84. I was in the Sonoran desert so the sun is very powerful. I took some temp readings as soon as I stopped. The shady side varied from 110 to 115. The sunny side was 135 on the outside dual and 115 on the inside. I think those are reasonable. In the desert it is not unusual to have static temps of over 100 before hiting the road.

[jadatis]

Originally Posted by moisheh

I am afraid that some of the info provided in this thread is not only confusing it may be incorrect. Jadatis: I am assuming you are in Europe as I don't think Pirelli sells commercial tires in NA. Regardless none of the 22.5 commercial tires are speed rated at 81 mph!! But I thank everyone for their assistance. I am going to reweigh when loaded. Yesterday I was on the road and the ambient temp was 84. I was in the Sonoran desert so the sun is very powerful. I took some temp readings as soon as I stopped. The shady side varied from 110 to 115. The sunny side was 135 on the outside dual and 115 on the inside. I think those are reasonable. In the desert it is not unusual to have static temps of over 100 before hiting the road.

The Pirelli document prooves that the 6945lbs is also used in Europe , the same as in America.
All tires over the world are calculated in their maximum load with the same system. This is done in KG and kPa ( 100kpa=1bar=14.5psi).
In the Pirelli document you see the different speeds of 130km=81m/h ( thanks for the conversion) and 120km/74m/h with 2 LI higher .
The 3150 given for 130km/h is conversed 6945 wich you gave for your tires at 131 psi wich is the conversion of 900kPa. So this prooves that your tires are the same calculated as the same sises and pressure written on sidewall as all tires , even those sold in Europe.

I once determined that the inside tire temperature warm is about 112degr F.
Mayby the treath you measure is a bit hotter with its 115 dgr but already about the same.
A valve importer once wrote me that the tire inside temperature can rise even up to 100dgr C/200dgr F incidentially by the heat of the brakes transported trough the rimms. Tires and valves are tested to stand the pressure-rising by this temperature rising .

Once you have done this 4 point weighing fully loaded you can fill in my spreadsheet . Placed it in this examples map under your name together with the pdf of it and the picture I placed in my last post.
https://skydrive.live.com/?cid=a526e...E092E6DC%21836
First download it by RIGHT-CLICKING and choose DOWNLOAD, then open it in Excel or Open Office CALC on your computer.
You stay boss with chanching the reserve percentages and the L% borders.
thats what I already used to work the advice pressure to 90 psi.

[Mr_D]

ON TRUCK size tires the cold pressure on the sidewall is the MINIMUM required to support the maximum weight rating of the tires. Same with the tire charts, it's the MINIMUM cold pressure to support the weight.

From page 2 of the 06/07 Michelin RV Tire Guide:

"If you look at the tire's sidewall, you'll see the maximum load capacity allowed for the size tire and load rating, and the minimum cold air inflation needed to carry the maximum load."

From the Firestone/Bridgestone RV tire guide:

Bear in mind that these are maximum ratings. The sidewall of the tire shows maximum load and minimum inflation pressure for that load

From the GoodYear RV Tire Guide:

How much air is enough?
The proper air inflation for your tires depends on how much your fully loaded RV or trailer weighs. Look at the sidewall of your RV tire and you’ll see the maximum load capacity for the tire size and load rating, as well as the minimum cold air inflation, needed to carry that maximum load.

From TOYO

Q: What are the consequences of inflating the tires to accommodate the actual loads?
A: If the inflation pressure corresponds to the actual tire load according to the tire manufacturer’s load and pressure table, the tire will be running at 100% of its rated load at that pressure. This practice may not provide sufficient safety margin. Any air pressure loss below the minimum required to carry the load can result in eventual tire failure.
But then they go ahead and publish a weight/pressure chart allowing lower pressure for RV's!!

[Mr_D]

Originally Posted by Craig36

My Pace Arrow 36B plate says 100 lbs in both front and rear. That is for an empty coach. It runs OK with 100, but at 102 handles much better and may give a tiny improvement in mileage. I'm sticking with 102. Max rating on my Michelins is 110 lbs.

Actually the tire pressures given on the manufacturers weight certificate are for the rig as loaded to it's maximum when using the OEM tires.
And again the pressure on the sidewall of an truck size RV tire is the MINIMUM for the maximum rating of that tire.

[moisheh]

Our MH was loaded today and I was lucky enough to find a scale where I could weigh one side. But not 2. Loaded the FA was 7500 lbs. and the RA was 15,820. If I were on a long trip probably would be about 300 more on the rear and maybe 100 on the front. Full fuel and full water. When I weighed one side it was 3500 FA and 7620 RA. If my math is correct I would call the FA 4000 times 2 or 8000 lbs. ( 7500-3500). The RA would be 8200 times 2 or 16,400(15820-7620). According to my tire chart 85 lbs would support 17640 on the rear (8820 per side) and the front would support 9570 ( 4785 per side). I hesitate to go to 80 lbs. as that is the minimum pressure for those tires. Any thoughts? I want to run lower pressures as the unit rides a little hard. I have new Koni's but have not installed them.

[Craig36]

One can go crazy with all this weighing and speculation. Your manufacture posted a plaque with the recommended pressure as delivered from the factory. Recognizing that your total added weight is likely less than 1000 pounds. Just add a couple extra pounds, (I add 2 extra) to cover the load. Now as to distributed load.... this changes as you load differently each trip. Also, a typical tire gage can be by up to 5-7%. On a 100 LB tire that's a lot of pounds, so best to always add extra. Your coach will handle better, and you will get better mileage.

[Mr_D]

Originally Posted by Craig36

One can go crazy with all this weighing and speculation. Your manufacture posted a plaque with the recommended pressure as delivered from the factory. Recognizing that your total added weight is likely less than 1000 pounds. Just add a couple extra pounds, (I add 2 extra) to cover the load. Now as to distributed load.... this changes as you load differently each trip. Also, a typical tire gauge can be by up to 5-7%. On a 100 LB tire that's a lot of pounds, so best to always add extra. Your coach will handle better, and you will get better mileage.

That "recommended" pressure is for the rig loaded to it's maximum, not as it weighed at the factory.

[FastEagle]

Originally Posted by Mr_D

That "recommended" pressure is for the rig loaded to it's maximum, not as it weighed at the factory.

I posted a couple of references in post #18. If you can take the time to go back and read them you will see that under normal circumstances it is never recommended to use less air pressure in your tires than has been recommended on the certification label. Deviations from the norm will be found in the vehicle owner's manual.

FE

[moisheh]

If you are using a cheap tire gauge then maybe 5 % is correct but a good quality Milton dual head will give readings that are less than 2% off. There are other good gauges that will give similar readings. Stay away from cheap Chinese junk.